1. Field of the Invention
The present invention relates to a scanning unit for an optical position measuring device.
2. Description of the Related Art
A scanning unit for an optical position measuring device is known from WO 99/08074 which, besides periodic incremental signals, also delivers at least one reference pulse signal at a defined reference position of a scale and at a scanning unit which can be moved in relation to the latter. At least one reference marker field is arranged on the scale and is integrated in an incremental graduation track, and the scanning device includes a detector arrangement which has at least three active detector areas arranged next to each other in the measuring direction. Two of the three detector areas constitute reference pulse signal detector areas and are used for generating a reference pulse signal, wherein the relative arrangement of the reference pulse signal detector areas in the measuring direction has been selected as a function of the structuring of the reference marker field on the scale. A detector area arranged between the reference pulse signal detector areas is used for generating at least one incremental signal.
As can be seen in particular in FIG. 3a of this publication showing an arrangement of active detector areas for the reference pulse signal areas, as well as for incremental signal detector areas, contact between the various active detector areas takes place at the front sides of the detector areas and, because of the many parallel line arrangements, the arrangement at the side of the detector areas requires a large amount of space on the printed circuit board, or on the semiconductor substrate of the photodetector, on which the active detector areas and the strip conductors making contact are to be arranged. Therefore this space required for the strip conductors on the side of the active detector areas cannot be used for the active detector areas on the semiconductor substrate for receiving the detector areas and the strip conductors.
However, it is of particular importance for creating the incremental and reference pulse signals to make the radiation-sensitive detector areas of the photodetector with the largest possible surface in order to couple in as much light as possible, which was received by the transmitted light or incident light methods, on the receiving side of the scanning unit. However, this is contraindicated by the miniaturization of the optical position measuring device, wherein the scanning unit should require the smallest possible volume, which results in that there is only a small surface available on the substrate for the detector areas. In connection with a relative and absolute measurement it is necessary to arrange the incremental signal detector areas, as well as the reference pulse detector areas, on this surface in such a way that a flawless detection and delivery of incremental and reference pulse signals is assured even under the most adverse conditions, such as heavy contamination.